Nikola Tesla (1856-1943) is, by all means, an iconic person for the modern world, and, as he lived a long life, he might be considered one of the cornerstone figures both for Steam and for Diesel era, though his passion was the Electricity. His heritage includes not only "mad science" common for Steampunk setting, but also very practical things the humanity uses for decades and will use for centuries in the future, barely knowing the origins of them - and I mean the electric motor here... Or wireless transmission, which he claimed successful a year before Alexander Popov and two years before Marconi.
Funny, but most of Tesla's practical inventions date back to Victorian Steam times, and growing older, to the Diesel age, he turned to sort of "mad inventor", ending his life in poverty and solitude of Waldorf-Astoria hotel room in New York, aged 86. His diaries still are not published in the entirety, and, who knows, maybe some amazing discoveries are hidden on that pages.
It's also useful to mention that Tesla, lacking fundamental knowledge in theoretical physics, was not the scientist per se, but an inventor and engineer. Also (and that was common for his era) he believed in the "Ether theory", which presumes that the air around us contains some substance, namely ether, in which electromagnetic waves are distributed and transferred. Modern science considers such approach a delusion, practically and reasonably denying ether existence, but the list of Ether theory advocates included not only Tesla but also famous Russian chemist Dmitry Mendeleev, who contributed the Periodical Table of Elements and many other things
You can read further biography in Wikipedia or in hundreds of books dedicated to this unconditionally exceptional man, and here I'd like to present one of devices he invented, which definitely suits the community right. Namely, it's Tesla Coil in action.